HAL Types

As part of the Android 8.0 rearchitecture of the lower layers of the Android OS
to support better modularity, devices running Android 8.0 must support binderized
or passthrough HALs:

Binderized HALs. HALs expressed in HAL interface definition
language (HIDL). These HALs replace both conventional and legacy HALs used in earlier versions of
Android. In a Binderized HAL, the Android framework and HALs communicate with each other using
binder inter-process communication (IPC) calls. All devices launching with Android 8.0 or later must
support binderized HALs only.

Passthrough HALs. A HIDL-wrapped conventional or legacy
HAL. These HALs wrap existing HALs and can serve the HAL in binderized and
same-process (passthrough) modes. Devices upgrading to Android 8.0 can use
passthrough HALs.

All other HALs provided by the vendor image can be in passthrough
OR binderized mode.

Binderized HALs

Android requires the following HALS to to be binderized on all Android
devices regardless of whether they are launch devices or upgrade devices:

android.hardware.biometrics.fingerprint@2.1. Replaces
fingerprintd which is no longer in Android 8.0.

android.hardware.configstore@1.0. New in Android 8.0.

android.hardware.dumpstate@1.0. The original interface provided
by this HAL could not be shimmed and was changed. Because of this,
dumpstate_board must be re-implemented on a given device (this is
an optional HAL).

android.hardware.graphics.allocator@2.0. Required to be
binderized in Android 8.0 so file descriptors don't have to be shared between
trusted and untrusted processes.

android.hardware.radio@1.0. Replaces the interface provided by
rild which lives in its own process.

android.hardware.usb@1.0. New in Android 8.0.

android.hardware.wifi@1.0. New in Android 8.0, replaces the
legacy Wi-Fi HAL library that was loaded into system_server.

android.hardware.wifi.supplicant@1.0. A HIDL interface over the
existing wpa_supplicant process.

NOTE: Android provides the following HIDL
interfaces which will always be in binderized mode:
android.frameworks.*, android.system.* , and
android.hidl.* (except for android.hidl.memory@1.0
as described below).

Passthrough HALs

Android requires the following HALs to be in passthrough mode on all Android
devices regardless of whether they are launch devices or upgrade devices:

android.hardware.graphics.mapper@1.0. Maps memory into the
process it lives in.

android.hardware.renderscript@1.0. Passes items in the same
process (equivalent to openGL).

All HALs not listed above must be binderized for launch devices.

Same-Process HALs

Same-Process HALs (SP-HALs) always open in the same process in which they are
used. They include all HALs not expressed in HIDL as well as some that are
not binderized. Membership in the SP-HAL set is controlled only
by Google, with no exceptions.

Conventional & legacy HALs

Conventional HALs (deprecated in Android 8.0) are interfaces that conform to a
specific named and versioned application binary interface (ABI). The bulk of
Android system interfaces
(camera,
audio,
sensors,
etc.) are in the form of conventional HALs, which are defined under
hardware/libhardware/include/hardware.

Legacy HALs (also deprecated in Android 8.0) are interfaces that predate
conventional HALs. A few important subsystems (Wi-Fi, Radio Interface Layer, and
Bluetooth) are legacy HALs. While there's no uniform or standardized way to
describe a legacy HAL, anything predating Android 8.0 that is not a conventional
HAL is a legacy HAL. Parts of some legacy HALs are contained in
libhardware_legacy,
while other parts are interspersed throughout the codebase.